Surprisingly large number of galaxies show evidence some kind of disruption. The question is how many of these disruptions create an apparent alignment with a background object.
Compare the number of objects actually found to the sizes of galaxy catalogs. At present, Arp's counts are almost certainly far lower than what you expect from chance alignments between foreground galaxies and background quasars.Some questions to think about when examining Arp's claims:
As a visual example of how perceptions can distort one's sense of statistics, consider the case of the galaxy UGC 10214, aka the Tadpole Galaxy (UGC 10214 data at Simbad).
|Examine this galaxy from a ground-based image.
What would you say is the probability of a quasar being aligned along the protrusion?
Image via Simbad POSSII/F/DSS2. This link will launch a Java Viewer application on your local machine.
|Now examine a subset of this view in this Hubble image taken with the HST/ACS. Notice all the background galaxies visible in this image which are almost unnoticeable in the image above.
Now what would you say is the probability of a quasar being aligned along the protrusion?
Higher resolution version of this image at STSci.
All the galaxies now visible in the background, many of which fall along the protrusion, significantly alters the view of just how many quasars may also fall along this track. Because any background quasars in any image are significantly brighter than the galaxies at the same distance, they are more likely to be visible in the more magnitude-limited image.
I've not found a paper that indicates the presence or lack of quasars along this stream. If anyone knows of such a paper, please .
So here is my current list of questions for Dr. Arp:
Here is another example. Consider the FULL Hubble field of view for the NGC 4319 and Mrk 205 pair and compare it on the same scale as the Hubble Deep Field image (Images from STSci).
|Hubble Deep Field||NGC 4319 and Mrk 205|
Arp's discordant redshifts are another product of the Malmquist bias. When we look at the night sky on Earth, the stars we see are not average stars, but the really bright ones very far away which can outshine the nearby fainter stars. While the bright stars are fewer in number relative to the average stars, the fact that they can be seen from a greater distance means we sample a larger volume of the sky, which more than makes up the difference.